There are a number of trade-offs in selecting a power source for a vehicle, particularly one that uses an internal combustion engine. Typically, a large displacement multi-cylinder engine (e.g. V-8, V-10, V-12) is employed where applications of high horsepower and/or torque are required or desired. Such engines can provide high power and torque quickly and are useful for rapid acceleration and towing power, among other things. A disadvantage of large-displacement engines is the lack of fuel economy. Such engines typically get lower mileage than small displacement engines (e.g. 2, 4, or 6 cylinder engines). A small displacement engine can provide greater fuel economy, but lacks high power and torque. Consequently, small displacement engines do not allow rapid acceleration, towing capacity, and the like.
One proposed solution for the trade-offs in selecting engine size has been the use of the so-called “variable-displacement” engine. Variable displacement internal combustion engines are known in the art to improve fuel economy by selectively shutting off fuel supply to several cylinders of the engine when reduced power output can operate the vehicle adequately. However, the current techniques used to implement variable-displacement engines create additional disadvantages.
One disadvantage is the technique of valve deactivation used in implementing a variable displacement engine. This reduces the number of active cylinders, but the deactivated cylinders still suffer from frictional losses of the deactivated chambers. Thus, a variable displacement engine operating on four cylinders does not have the same fuel efficiency as a stand-alone four cylinder engine. Therefore, the maximum efficiency of the variable displacement engine is not achieved.